The answer
If you are replacing a window, read the size code on the data plate fixed to the frame, which you can see with the sash open. A new window is sized to the room and the roof: the wider the space you want to light, the bigger the glass, within the gap between rafters and the height of the slope. Where the window must also be a way out in an emergency, the clear opening sets a minimum size.
Replacing a window: read the data plate
The quickest route to the right size is the old window itself. Open the sash fully and look at the frame, usually on the side or head: you will find a plate with the manufacturer's size code.
On VELUX windows the code pairs a letter group with two digits. The letters stand for the width and the digits for the height, so a window marked MK04 measures 78 cm wide by 98 cm tall. Other makers use their own systems, and the plate often also lists the type of glazing and the date of manufacture.
With the code in hand, a like-for-like replacement is straightforward, because the new window is made for the same opening and flashing. We still check the roof when we come, since flashings, battens and the surrounding tiles or slates may also need renewing. Our roof window installation in Nottingham page describes what a survey covers.
If the plate is painted over, unreadable or missing, measure the frame instead. Measure the outer frame width and height in centimetres and send us a photograph of the window from inside and out. We confirm the size on site before anything is ordered.
New window: start with the rafters
When there is no window already, the roof sets the limits. Rafters in a typical pitched roof run at regular spacing, and the easiest opening is one that fits the gap between two of them, because no timber needs cutting.
That gap is narrower in many older Nottingham houses, where timber was set to suit the material available, and spacing can vary along the roof. A wider window means cutting one or more rafters and building a frame around the opening, with trimmers across the top and bottom to carry the load. It is routine work, but it needs to be done properly and it adds time to the job.
The height of the slope matters too. A tall window on a short slope can run out of roof, leaving too little space for the flashing above and below. On a long slope you have room for a larger window or two stacked in a column.
Both things are checked in the loft, where the rafters can be seen and measured, and from outside, where the covering and gutters show the limits. It is the main reason we survey before we price. A size picked from a catalogue and not checked against the roof sometimes needs revising on site.
Sizing for the room and the light
After the roof, size for the room. As a rough guide, the larger and deeper the room, the more glass it can use, but position often matters more than area. A window in the right place lights a room better than a bigger one in the wrong place.
In a Nottingham winter most of your daylight arrives under cloud, which is brightest overhead. That works in favour of roof glazing, since it faces the brightest part of the sky, yet it also means a small window gives little to a big room. Rooms that stay dim in November usually need either a wider window or two windows spread across the room.
No English regulation sets a daylight minimum for homes, so the size is your call within what the structure allows. A bedroom may be happy with one window. A loft office or a kitchen-diner with a deep plan usually wants more.
Remember the inside as well. A deep, angled splay between the glass and the ceiling spreads the light, and a shallow one concentrates it. A white finish helps either. For more on how light behaves in a dark space, see our guide to choosing a skylight for a dark room.
When size is fixed by safety or energy rules
Two rules can set a floor on the size. The first is escape. If a window in a bungalow or chalet loft is the way out of a bedroom, it needs a generous clear gap, 0.33 square metres or more with no side under 450 mm, set low enough to climb through. The rule has its own page, on using a roof window as a fire escape. A window chosen only for its frame size may fall short on the clear gap.
The second is heat loss. Larger windows lose more heat in winter, so a bigger size makes the glazing specification matter more. Part L sets a ceiling of 1.4 W/m²K for a roof window in an existing home, or an energy rating of band B, and a good coated double-glazed unit meets it comfortably at any size.
Ask for the installed size including the frame, and look at how the finish reads from the street. Matching the size of existing windows on the same roof keeps a house looking tidy, which is also what councils tend to prefer on a visible slope.
Planning is simple for most houses: permitted development covers most rooflights, as long as they do not protrude more than 15 cm from the slope. Conservation areas and flats are different, and we check first.
What we do at the survey
We come to the house, look at the roof inside and out, measure the gap between the rafters and read any existing data plate. Then we talk through two or three sizes for the room and show how each sits on the slope.
The survey is free. Pricing waits until we have looked at your roof. The quote is then fixed and written down, so whichever size you agree is the figure you pay. Where the job is notifiable, the Building Control notification is ours to send, and the window is installed to current Building Regulations. A 10-year workmanship guarantee in writing covers our fitting.
If you are weighing a conversion, the sizing question joins up with where windows go on each slope. That is covered in roof windows for loft conversions.